1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly, to a method of driving a plasma display panel enabling to improve a contrast characteristic of the plasma display panel.
2. Discussion of the Related Art
Generally, a plasma display panel(hereinafter abbreviated PDP) is a device that displays images including characters or graphics by making phosphors emit light by a UV-ray radiating from the discharge of inert mixed gases(He+Xe, Ne+Xe, or He+Xe+Ne).
Such a PDP is advantageous in thinning its thickness and widening its screen size, and provides a greatly improved quality of image due to the recent development of technology.
It is typical that the PDP including 3-electrodes is driven by AC voltage. And, such a PDP is called an AC surface discharge type PDP.
In a 3-electrodes AC surface discharge type PDP, wall charges are accumulated on a surface on discharge of the PDP and the electrodes are protected from sputtering generated from discharge. Hence, the 3-electrodes AC surface discharge type PDP has advantages of low-voltage drive and long endurance.
A discharge cell of a 3-electrodes AC surface type PDP according to a related art includes scan and sustain electrodes Y and Z on a front substrate and an address electrode X on a back substrate. In this case, the address electrode X extends in a direction crossing with the scan and sustain electrodes Y and Z.
A front dielectric layer and a protective layer are stacked on the front substrate having the scan and sustain electrodes Y and Z running in parallel with each other. Besides, wall charges generating from the plasma discharge are accumulated on the front dielectric layer.
The protective layer prevents the front dielectric layer caused by the sputtering generated from the plasma discharge as well as increases a discharge efficiency of secondary electrons. And, the protective layer is generally formed of MgO.
Meanwhile, a back dielectric layer and a barrier rib are formed on the back substrate having the address electrode X. And, phosphors are coated on surfaces of the back dielectric layer and barrier rib.
The barrier rib lies in parallel with the address electrode X to prevent optical or electric interference between adjacent cells on the back substrate. Namely, the barrier rib prevents UV and visible rays, which are generated from the discharge, from leaking into the adjacent discharge cells.
The phosphors are excited by a UV-ray emitted from the discharge to emit a red, green, or blue visible ray. Inert mixed gases(He+Xe, Ne+Xe, or He+Xe+Ne) are injected in a discharge space provided between the two substrates and barrier rib.
The above-explained discharge cell has the electrodes arranged like a matrix form. A plurality of scan electrodes Y1 to Ym and a plurality of sustain electrodes Z1 to Zm are arranged in parallel with each other in discharge cells. And, the discharge cell is provided on each of intersections between the two parallel electrodes(Y1 to Ym and Z1 to Zm and address electrodes(X1 to Xn).
The scan electrodes Y1 to Ym are driven sequentially, while the sustain electrodes are driven I common. And, the address electrodes X1 to Xn are divided into odd and even lines to drive.
A drive time for representing a specific gray scale for a single frame in such a 3-electrodes AC surface discharge type PDP is separated into a plurality of sub-fields. And, light emission in proportion to a weight of a video data is carried out during each sub-field duration to perform the gray scale.
FIG. 1 illustrates a constructional diagram of a frame in accordance with a PDP drive according to a related art.
Referring to FIG. 1, a single frame according to a drive of a 3-eletrodes AC surface discharge type PDP is divided into a plurality of sub-fields by time. Specifically, a single frame is divided into various sub-fields differing in the number of light emissions to drive with time-division.
Each of the sub-fields SF is divided into a reset period for resetting an entire screen, an address period for selecting a scan electrode line and selecting discharge cells on the selected scan electrode line, and a sustain period representing a gray scale according to the discharge number for the discharge cells selected by an address discharge.
For instance, when an image is displayed with 256 gray scales using 8 bits video data, as shown in FIG. 1, a frame period(16.67 ms) corresponding to 1/60 second is divided into eight sub-fields SF1 to SF8. Each of the eight sub-fields is driven for the reset period, address period, and sustain period. In this case, the reset and address periods are set up to have the same rate for each of the sub-fields. On the other hand, the sustain period of each of the sub-fields is given thereto with a time weight having a rate of 2N(where N=0, 1, 2, 3, 4, 5, 6, and 7). Namely, the sustain periods increase with the rates of 1:2:4:8:16:32:64:128 from the first sub-field SF1 to the eighth sub-field SF8, respectively.
FIG. 2 illustrates a diagram of a drive waveform according to a PDP drive in the frame in FIG. 1, in which ‘Y’, ‘Z’, and ‘X’ indicate scan, sustain, and address electrodes, respectively.
Referring to FIG. 2, each sub-field of a PDP according to a related art is divided into a reset period for resetting an entire screen, an address period for selecting a cell, and a sustain period for maintaining a discharge of the selected cell to drive.
The reset period is separated into a set-up period and a set-down period. A reset pulse having a ramp-up waveform is simultaneously applied to scan electrodes during the set-up period, and the other reset pulse having a ramp-down waveform is applied thereto during the set-down period.
During the set-up period SU of the reset period, the rest pulse RP of the ramp-up waveform is applied to the scan electrodes Y. And, a set-up discharge occurs in the discharge cells of the entire screen by the reset pulse RP of the ramp-up waveform. Positive(+) wall charges are then accumulated on the address and sustain electrodes X and Z by the set-up discharge, while negative(−) wall charges are piled up on the scan electrodes Y.
Subsequently, the reset pulse -RP of the ramp-down waveform is applied to the scan electrodes Y during the set-down period SD. The reset pulse -RP of the ramp-down waveform has a waveform descending from a positive voltage lower than a peak voltage of the reset pulse RP of the ramp-up waveform after the reset pulse RP of the ramp-up waveform is applied thereto.
The reset pulse -RP of the ramp-down waveform brings about a weak erase discharge(i.e. set-down discharge) in the discharge cells to erase the wall charges, which are piled up on the respective electrodes X, Y, and Z excessively, in part as well as unnecessary charges in space charges. Hence, the wall charges amounting to the extent that enables the set-down discharge to trigger stably the address discharge remain in the discharge cells uniformly.
While the reset pulse -RP of the ramp-down waveform is applied to the scan electrodes Y, a positive(+) DC(direct current) voltage DCSC is applied to the sustain electrodes Z. Namely, at the time point that the reset pulse -RP of the ramp-down waveform is applied, the positive(+) DC voltage DCSC starts being applied to the sustain electrodes Z. And, the DC voltage DCSC is maintained until the rest pulse -RP of the ramp-down waveform reaches a negative(−) reset-down voltage Vrd, and is kept being applied during the subsequent address period.
While the DC voltage DCSC is applied to the sustain electrodes Z during the address period, a negative(−) scan pulse SP is applied to the scan electrodes Y and a positive(+) data pulse DP synchronized with the negative(−) scan pulse SP is applied to the address electrodes X.
As a voltage difference between the scan pulse SP and the data pulse DP is added to the voltage by the wall charges generated from the reset period, an address discharge occurs in the discharge cell supplied with the data pulse DP.
In the discharge cells selected by the address discharge, wall charges enough to generate the discharge are formed when the sustain voltage is applied thereto.
In order to generate a sustain discharge from the discharge cell selected by the address discharge, sustain pulses SUSPy and SUSPz are applied to the scan and sustain electrodes Y and Z alternately.
In the discharge cell selected by the address discharge, a sustain discharge, i.e. display discharge, is generated between the scan and sustain electrodes Y and Z whenever the sustain pulses SUSPy and SUSPz are applied thereto as the voltages by the sustain pulses SUSPy and SUSPz are added to a wall voltage causes by the wall charges in the discharge cell.
After completion of the sustain discharge, an erase pulse of a ramp wavelength(not shown in the drawing) having a small pulse width and a voltage level is applied to the sustain electrode Z to erase the wall charges remaining in the cells of the entire screen.
When the erase pulse is applied to the sustain electrode Z, a voltage difference between the sustain and scan electrodes Z and Y increases gradually to bring about weak discharges between the sustain and scan electrodes Z and Y consecutively. In this case, the weak discharge erases the wall charges existing in the cells where the sustain discharge has occurred.
However, the PDP according to the related art decreases its contrast characteristic since the wall charges are excessively formed on the scan and sustain electrodes Y and Z during the reset period.
Such a problem is fully explained in detail by referring to FIG. 3. FIG. 3 illustrates a diagram of wall charge formation of set-up and set-down periods according to a square waveform in FIG. 2.
First of all, when the reset pulse RP of the ramp-up waveform applied to the scan electrodes Y is applied during the set-up period SU, the set-up discharge occurs in the discharge cells of the entire screen. Hence, as shown in a part A, the negative(−) wall charges are formed on the scan electrodes Y while the positive(+) wall charges are formed on the sustain and address electrodes Z and X.
Subsequently, since the polarities of the wall charges formed on the respective electrodes are inversed by the reset pulse -RP of the ramp-down waveform applied to the scan electrodes Y and the positive(+) DC voltage DCSC applied to the sustain electrodes Z during the set-down period, the wall charges generated excessively and irregularly, as shown in part B, are reduced to a predetermined quantity.
After the end of the reset period, the negative(−) scan pulse SP applied to the scan electrodes Y and the positive(+) data pulse DP applied to the address electrodes X for synchronization with the scan pulse SP reciprocally are added to the voltage generated by the wall charges accumulated previously during the set-down period SD, whereby the address discharge occurs in the discharge cell supplied with the data pulse DP.
In this case, the discharge between the scan and sustain electrodes Y and Z is generated at a voltage lower than that between the scan and address electrodes Y and X. Thus, an emission amount of light proceeding toward an observer exceeds that of the other light generated by the discharge between the scan and address electrodes Y and X, whereby the emission amount of the light for the reset and address periods as the non-display period of gray scale increases. Hence, the contrast characteristic is degraded as much as the increment of the emission amount of the light.
In order to prevent the degradation of the contrast characteristic, it is preferable that the address discharge as the non-display discharge between the scan and address electrodes Y and X is generated in a vertical direction. Yet, as the voltage difference between the scan and sustain electrodes Y and Z is added to the voltage of the wall charges generated for the reset period, the discharge is generated between the scan and sustain electrodes Y and Z in a surface direction. Therefore, the degradation of the contrast characteristic is inevitable since the light by the discharge generated between the scan and sustain electrodes Y and Z in the surface direction is barely generated from the entire area of the discharge cell.